Wave amplifying system



Jan. 30, 194.0. I; 0. WILSON 2,188,671

WAVE AMPLIFYING SYSTEM Filed March 29, 1938 FIG./

IN TERS TAGE AND OUTPUT NETWORK cmcq/r I I Q SULPI-IIDE 5 EQUALIZER.

SULPHIDE PILOT WIRE PILOT REGULA TOR CHANNEL CONTROL [6 /6 3 mrsnsrncz: i u I nvrmsmcs M0 OUTPUT 7 NETWORK cmcu/r L SIL van sum/4105 /5' & J /3 N SILVER SULPH/DE f EQUAL/ZEN PILOT 522%214 CONTROL 22 CON TROL I IN I/E N TOR B [6. ML SON ane A T TORNE V Patented Jan. 30, 1940 UNITED. STATES PATENT OFFICE Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application March 29, 1938, Serial No. v198,t19 15 Claims. (Cl. 179- 171) This invention relates to wave amplifying systems, as for example, vacuum tube amplifiers.

Objects of the invention are to control transmission properties of such systems, for example, 5 gain and distortion introduced by the, systems, to facilitate application of feedback in the systems, and to reduce singing tendency ormaintain proper. margin against singing in the systems.

In one specific aspect, the invention-is applied to an amplifier with a forwardly transmitting, wave amplifying portion and a feedback path from the output to the input of the amplifying portion for producing negative or gain-reducing feedback around the amplifying portion, for ex,- ample, to stabilize the gain or reduce the modulation introduced by the amplifier, in accordance with the principles described by H. S. Black in his article entitled Stabilized feed-back ampli fiers, Bell System Technical Journal, January 0 1934, and in his Patent 2,102,671, Decembery2l,

With sufiicient feedback, the gain of such/an amplifier varies substantially as the loss in the feedback path, and is substantially independent of the amplification ratio or transmission efl'iciency of the amplifying portion. It is usual to control the amplifier gain for any given frequency by changing the loss in the feedback path for such frequency.

However, this change causes variation in the amount of feedback or the loop gain of the amplifier (i. e., the gain in decibels for propagation once around the feedback loop), and the variation in the loop gain may be objectionable. For

example, it may unduly reduce the singingmargin or the amount of modulation reduction or amplifier gain stability obtained by the feedback action, or objectionably change the amplifier input or output impedance. I 1

A feature of thepresent invention is provision of means for stabilizingthe amount of. feedback or the loop gain, fora given frequency, against variation due to change produced in transmission efficiency of the feedback path. For; eX-- ample, such change may be a given decreasemade in the transmission efiiciency of the feedback path in order to increase the gain of the amplifier,

and operation of the loop gain stabilizing means may increase the transmission eiliciency of the amplifying portion of the amplifier substantially the given amount in order to reduce variation of the loop gain or maintain the amount of feedback substantially unchanged, as for example, to maintain the feedback at the maximum value that is permitted by the required margin against singing; or the change in the transmission efficiency of the feedback path may be a given increase made in order to decrease the gain of the amplifier, and then operation of the loop gain stabilizing means may insure decrease of the 5 transmission efficiency of the amplifying portion of the amplifier substantially the given amount in order to reduce variation of the loop gain or maintain the loop gain substantially unchanged, for instance at the maximum value that is per- 10 mitted by the required singing margin.

If desired, the inverse or complementary changes in transmission efiiciency of the feedback path and transmission efficiency of the amplifying portion of the amplifier may be made autol5 matically, as for example by a pilot wire or pilot channel gain control or an automatic volume control. For example, the automatic gain control may vary the temperature (and thereby the resistance) of a heat responsive resistance (ther-- 2 0 mistor), for instance a silver sulphide resistor in a shunt arm of the feedback path, and the tem- 'perature (and thereby the resistance) of a silver I sulphide resistor in series in the grid-cathode.

and plate-cathode circuits of a vacuum tube in- 25 cluded in the amplifying portion of the amplifier, to produce the desired changes in the transmission efficiencies of the forward and feedback paths. 7

Other objects and aspects of the invention'will 30 be, apparent from the following description and claims. Figs. 1 and 2 show circuit diagrams of two forms of the invention.

The amplifier of Fig. 1 may be a stabilized feed- 35.

back amplifier of the general type disclosed, for example, in the above-mentioned patent and published article. It comprises an amplifying path or element shown as including tandem connected vacuum tubes l and 2 and interstage and output circuit 3 which may include one or more stages of vacuum tubes. It also comprises a feedback path 1 shown as including a transmission control network. N. The amplifying path or element, that is, the forward path including thev elements 5 l 2 and 3, may be referred to as the circuit and the feedback path may be referred to as the B-circuit, the significance of ,u, ,3, .a-Cll'Cllil? and ,c-circuit being as indicated in the patent and. published article, justjmentioned, The network 50 N may be referred to as the ,B-circuit network. The quantity s, the propagation around the closed feedback loop, or the modification that a voltage undergoes in transmission once around the closed feedback loop, may be large compared 55 to unity, as for example of the order of- 50 or and 1,, the propagation of the forwardly transmitting path, may be large compared to ,s, the propagation of the feedback path.

The network N may be any suitable transmission control network, for example for amplitude or phase equalization or correction of distortion in the general manner disclosed in the patent and published article just mentioned. For instance, it may be a transmission equalizing network such as network 374 in Fig. 65 of that patent or in Fig. 4 of H. S. Black Patent 1,956,547, May 1, 1934, or network 14 of F. A. Brooks Patent 2,075,975, April 6, 1937, network N of E. B. Payne Patent 2,075,956, April 6,1937, or network 27 of R. W. Chesnut Patent 2,154,062.

The amplifier receives wavesfrom an incoming line or circuit 5 terminated in amplifier input transformer 6 and transmits the amplified, Waves to outgoing line or circuit I. The circuits 5 and I may be, for example, sections of a multiplex carrier cable or open wire circuit. For instance, the amplifier may amplify simultaneously the waves of a number of carrier telephone or telegraph channels, or both, extending over a wide frequency range.

The interstage circuit coupling tubes I and 2 comprises interstage network 8 and a shunt arm or branch including a thermistor or thermosensitive element 9, as for example a silver sulphide resistor. Associated with resistor 9, but electrically insulated therefrom, is a heating element I0 therefor. The'feedback path 1 comprises a shunt arm including a resistor IE and comprises a series arm including a thermosensitive element I2, as for instance a silver sulphide resistor. Associated with resistor I2, but electrically insulated therefrom, is a heating element I3 therefor. Elements 9 and I9 are shown in a heating chamber, I4; and elements I2 and I3 are shown in a heating chamber '15. Current for heating the elements l0 and I3 to vary the resistance of elements 9 and I2 is suppliedfrom a power source in a regulator control circuit, for example a pilot wire regulator control circuit ZI when switch 22 is closed to the left, or a pilot channel regulator control circuit 23 when the switch is closed to the right. The pilot control 2I is, as usual, responsive to resistance changes in a pilot wire or loop circuit, indicated by pilot conductors 25, which may be, for example, a pair 1 of wires that is included in the same lead cable sheath as the signal conductors such as 5 and extends over the length of the line section assigned to the amplifier and is subject to the same temperature variations as the signaling conductors.

With switch 22 closed to, the left, increase of attenuation of the line 5 and the pilot wireloop circuit 25 causes the control 2| to decrease the current transmitted to heaters I3 and II], for increasing the resistances of elements I2 and 9 simultaneously. The increase of resistance I2 increases the loss or reduces the transmission efficiency of the feedback path I or the ,B-circuit of the amplifier, or in other words reduces the magnitude of B, and thereby increases the amplifier gain to compensate for the increased attenuation of line 5 caused by the increase of temperature of the line and thus maintain a constant transmission equivalent for the line and amplifier; and the increase of resistance 9 decreases the lossor increasesthe amplification of the, forward path or l-circuit of the amplifier, pref-.

erably by the same amount that the loss of the p-circuit increases, so that the magnitude "of p 3 remains unchanged.

By additional impedance arms, for instance as shown by Z and Z, controlled by resistances I2 and 9, changes in the phase shift of p, that is,

in the phase shift around the over-all feedback 9 loop, may be introduced.

For exaniplevariation of the resistance I2 may produce a substantially fiat amplifier gain change across the used frequency band but due to stray capacity produce undesired loop phase shift outside that band. By connecting Z around re- Q sistance 9 and suitably proportioning the constants ,of impedance Z, the deleterious phase shift outside of the used frequency band may be annulled at the same time that resistance 9 is varied when obtaining the desired amplifier gain settings in the used frequency band.

Further,if by the combination of Z and I2 an amplifier gain characteristic is obtained which is not flat across the used frequency range, then as I2 is varied the m3 gain and phase (i.'e., the;

gain and phase around the over-all feedback I loop) may tend to'change materially. By prop'-' erly designing the impedance Z' connected across resistance 9, compensatory and opposite change of phase shift, as well as of gain, can be introduced by 9 in transmission through the -circuit, that is the forward path of the amplifier. In this Way, the 8 gain and phase shift (i. e., the gain and phase shift around the over-all feedback loop) may bev maintained constant in the working frequency range, for various tempera tures or settings of I2. I

If desired, the networks Z and Z may be designed to give constant loop phase shift, i. e;,

constant phase angle of propagation around the so that the c-circuitloss and the -circuit gain decrease equally'to maintain constancy of the transmission efi'lciency around the feedback loop,

and at the same time to decrease the over-all amplifier gain and thereby maintain constancy of the transmission equivalent of the line and amplifier.

'The control 2I may be of any suitable type. For example, it may be of the general typedisclosed in Mallinckrodt Patent 2,098,968 November 16, 1937; or the controlZI maybe, for example, a power source and a heater current ad 1 justing rheostat such as the power source and 1 the heater current'adjustin pilot apparatus controlled rheostat T of Fig.3 of E. I. GreenPatent 1,918,390, July 19, 1933, and'a pilot wire trans-' mission regulator control equipment for operating the rheostat, suchiasthat which operates the gain adjusting rheostat 12 of the above-mentioned H. S. Black Patent 1,956,547.

With switch 22 closed to the right, changeof' attenuation of line 5 causesv control 23, instead of control 2|, to operate and the results of such operation arethe same as those indicated above for operation of control 2|.

'The control 23 may be of any suitable type. For example, it may be of the. type disclosed in the above-mentioned E. 1. Green Patent 1,918,390 or of the type disclosed in R. 'W. Chesnut Patent "2,154,062, April ll 19s9;- or mmay be -of the type disclosed in Fig. 2 of H. S. Black Patent 2,154,888, April-18, 1939. H I I The energizing circuits such as the plate current supply-, grid biasing and cathode heating circuits for the amplifier are omitted from the drawing in the interest of simplicity-as they may be of any usual or suitable type.

2 shows a modification of the circuit of Fig. 1 in which theamplifier gain control resistor iS'fl resistor l2 in a shuntarm of the p-circuit or feedback path, and in whichthe rethereby controlling the gain of thestage comprising this tube. I Y

A resistor I 3' in a heating chamber 15 with resistor I 2- acts as a heating element therefor.

A resistor It in a heating chamber M' with resistor 9 acts as a heating element therefor.

The operation of the system of Fig. 2 is generally similar to that of Fig.1 and will be apparent from the description above of the operation of the latter system. It is to be noted, however, that since gain'control resistor I2 is in a shunt arm of thefeedback path f, and since increase of resistance '9 reduces the transmission efficiency of the forward path around which the feedback path f is connected, in case ele-,

ments l2 and 9' have negative temperature coefficients of resistance, increase of the line attenuation should be accompanied byincrease of the current transmitted to the. heaters l3" and Ill, for decreasing the resistances of elements l2 and 9' simultaneously and thereby maintaining the magnitude of s constant and maintaining the transmission equivalent of the line and amplifier constant. Similarly, decrease of the attenuation of line 5 should be accon1- panied by decrease of the current to the heaters 43' and I0, so that the fi-circuit loss and the -circuit gain decrease equally to maintain constancy of the transmission emciency around the feedback loop including'the forward path and the feedback path f, and at the same time to decrease the over-all amplifier gain and thereby maintain constancy :of the transmission equivalent of the line and amplifier.

The control M maybe of any suitable type. For example, it may be of the type disclosed in the above-mentioned Mallinckrodt'Patent 2.698,-

968 or it may be current source and rheostatv Fi 2 of H. S. Black Patent 2,154,888.

What is claimed is:

1. The method of operating on a negative feedback amplifier which comprises changing the over-all amplifier gain and making the magnitude and frequency variations of the gain around the feedback loop the same for the changed overall gain as for the original over-all gain.

2. The method of operating on a negative feedback amplifier which comprises changing "the over-all amplifier gain and making the -magni-',

tude and frequency variations of the gain and phase shift around the feedback loop thesame for the changed over-all gain as for the original 545 over-all gain. I r

3. The method of operating'a negative feedback amplifier which comprises changing the propagation of the feedbackpath at a given frecompensatory substantially equal but inverse quency and rendering the propagation around 2119. the feedback loop under the changed condition substantially the same as before saidchange by change in the propagation of the forward path around which the feedback path is connected.

4. The method of operating a negative feedback amplifier which comprises changing-the phase shift of the'feedbackpath at a given frequency and rendering the phase shift around the feedback loop under the changed condition sub- 5&0

stantially the same as before said change by compensatory substantially equal but inverse'change in the phase shift of the forward'path around which the feedback path is connected.

5. A negative feedback amplifier with means for changing the transmission efficiency of the feedback path at a given frequency, and means whose operation insures change of the transmission efficiency of the forward path, around which the feedback path is connected, inversely and by {59 substantially the same amount.

6. The method of operating a negative feedback amplifier which comprises changing the loss and the phase shift of the feedback path at the phase shift around the feedback loop under the changed condition substantially the same as before said change by compensatorychange in the gain and the phase shift of the forward path around which the feedback path is connected.

'7. A negative feedback amplifier comprising means for changing the over-all amplifier gain whose operation tends to change the'feedback,

9. A negative feedback amplifier with meansto increase the transmission efficiency of the feedback path for reducing the amplifier gain, means to correspondingly decrease the transmission efficiency of the forward path around which the feedback path is connected and thereby make the magnitude and frequency variation of the gain around the feedback loop the same as before said increase'and decrease, and means comprising common operating means for said two first-mentioned means for causing said increase and said decrease to take place together.

10. A wave translating system comprising an" amplifier having a forward amplifying path transmitting a broad frequency band of waves and a feedback path connected around said forward path for producing reverse feedback in said amplifier, a network in said feedback path having av transmission characteristic varying with a given frequency and rendering the gain and =frequencyover said band for controlling the overall, amplification of said amplifier, and

means operable for inversely varying the transmission efliciencies of said paths and causing the variation of the transmission efficiency of said amplifying path to substantially equal in magnitude the variation of the transmission efiiciency of said feedback path.

11. A negative feedback amplifier with gain control means comprising a temperature dependent impedance in the feedback path, transmission efficiency control means comprising a temperature dependent impedance in the forward path around which the feedback path is con- .nected, and means coordinately controlling the temperatures of said impedances and thereby causing variation of their impedance values that produces substantially equal but inverse changes in the transmission efficiencies of said paths.

12. In a gain control system, a negative feedback amplifier with gain control means comprising a'heat-responsiveresistor in the feedback path and transmission control means comprising amplifier comprising a forward amplifying path and a feedback path connected around said forward path for producing negative feedback in 1 said amplifier, a silver sulphide resistor in a shunt arm of said feedback path, means for varying the transmission 'efilciency of said forward, path without materially affecting the transmission efiiciency of said feedback path including a silver sulphide feedback resistor producing ina portion of said forward path negative feedback that increases with increase'of the resistance of said feedback-resistor, current responsive means for heating said resistors, and an automatic gain regulator control circuitfor supplying said curn -10 gain control system, an

rent.

14. In an automatic amplifier comprising a forward amplifying path and a feedback path connected around said for ward path for producing negative feedback in said amplifier, a silver sulphide resistor in a series arm of said feedback path, means for varying the transmission eificiency of said forward path without materially affecting the transmission efficiency of said feedback path'including a silver sulphide resistor in a shunt arm of said forward; path, current responsive means for heating said resistors, and an automatic gain regulator coni trol circuit for supplying said current.

15. The method of operating a negative feedback amplifier having the transmission efiiciency of the feedback path vary over. a range of fre- I quencies, which comprises changing the transmission efficiency and the phase shift of the feed- 7 back path at frequencies in said. range and rendering the transmission efiiciency and the phase shift around the feedback loop under the changed condition substantially the same as before said change bycompensatory"substantially equal but inverse change in the transmission efiiciency and the phase shift of the forwardpath around which the feedback path is connected. r

IRA G. WIL- son.. 

